Anthony Wesley was engaged in his favourite backyard pastime of watching Jupiter through his 14.5-inch-wide Newtonian telescope when the amateur Australian astronomer made the discovery of a lifetime - a find that has astonished and enthralled professional planet watchers around the world.

It was just after midnight on Sunday and Mr Wesley, a 44-year-old IT consultant living in the rural town of Murrumbateman near Canberra, was about to call it a night after the clear skies he had enjoyed that evening began to deteriorate and visibility fell.

But having gone inside and got caught up watching golfer Tom Watson almost make history in the Open on television, something made Mr Wesley return to the computer screen of his home-made telescope once more.

It was then he saw the dark disfiguration of a region near to Jupiter's south pole, a part of the planet that he knew well from his many hours of painstaking observations.

"When I came back to my escope at about 12.40am, I noticed a dark spot rotating into view in Jupiter's south polar region [and] started to get curious. When first seen... it was only a vaguely dark spot, I thought likely to be just a normal dark polar storm," Mr Wesley recorded in his observation report.

"However, as it rotated further into view, and the conditions improved, I suddenly realised that it wasn't just dark, it was black in all channels, meaning it was truly a black spot," he said.

His first thought was that it was one of Jupiter's darker moons, such as Callisto, or at least the shadow from such a moon.

"But it was in the wrong place and the wrong size. Also, I had noticed it was moving too slow to be a moon or shadow," he said.

"As far as I could see it was rotating in sync with a nearby white oval storm that I was very familiar with - this could only mean that the black feature was at the cloud level and not a projected shadow from a moon. I started to get excited."

For the next 15 minutes Mr Wesley furiously checked his photographs of the same region taken just two days before. There was no sign of the black spot.

It was then he realised the full magnitude of his discovery n he had witnessed the immediate aftermath of a cosmic collision between Jupiter and some as-yet-unidentified object.

Mr Wesley remembered one of the most famous cosmic impacts in recent times, when the comet Shoemaker-Levy pounded Jupiter exactly 15 years ago with the explosive power of thousands of nuclear bombs.

"I remember watching Jupiter back then, so I grew up with those images," he said.

"Could it really be an impact mark on Jupiter? I had no real idea, and the odds on that happening were so small as to be laughable, but I was really struggling to see any other possibility given the location of the mark. If it really was an impact mark then I had to start telling people, and quickly," he said.

The isurreali experience of seeing the same sort of image again spurred him into action. He quickly fired off emails to the international astronomical community and the US National Aeronautics and Space Administration (Nasa).

Glenn Orton of Nasa's Jet Propulsion Laboratory in California used a high-powered infrared telescope on a mountaintop in Hawaii to confirm the sighting. The dark spot was accompanied by bright 'upwelling particles' in the upper atmosphere of Jupiter, with the possible emission of ammonia gas, - the tell-tale signs of a massive impact.

It was clear that Jupiter had been hit, although Dr Orton and his Nasa colleagues were not sure whether it was by a comet or some other celestial object. "It could be the impact of a comet, but we don't know for sure yet," Dr Orton said.

"We were extremely lucky to be seeing Jupiter at exactly the right time, the right hour, the right side of Jupiter to witness the event," he said.